The present invention relates to radio frequency (RF) mixers.
Radio receivers typically receive an RF signal and downconvert it to a signal having a lower frequency, which is easier to amplify, filter and process. This is usually accomplished in a mixer that mixes the RF signal with local oscillating (LO) signal having a different frequency. The mixer then outputs an intermediate frequency (IF) signal that is further processed by the receiver.
Similarly, a radio transmitter typically receives an IF signal and upconverts it to a signal having a higher, RF frequency for transmission. This is usually accomplished in a mixer that mixes the IF signal with a LO signal having a different frequency. The mixer then outputs a RF signal.
Mixing is commonly used in communication systems, such as in cellular communications and cordless telephony. For example, a handset receives a RF signal and downconverts the signal via a mixer to an IF signal. It is important that the mixer be low noise so that it does not significantly degrade or mask the information contained in the original RF signal. It is also important, particularly for cellular handsets, that the mixer consumes low power. Moreover, the mixer should be easy to manufacture, preferably as an integrated circuit.
Mixing techniques used to date, however, have shortcomings in performance and/or in implementation as an integrated circuit. For example, a conventional balanced integrated circuit mixer is shown in FIG. 1. This mixer is commonly referred to as a Gilbert cell or Gilbert mixer. The RF signal is input to a balun 10, which outputs a balanced RF signal (the original RF signal RF+ and an inverted RF signal RFxe2x88x92). The RF+ signal is input to the base of transistor Q1 and the RFxe2x88x92 signal is input to the base of transistor Q2. The emitters of transistors Q1 and Q2 are coupled to a current source IEE. The collector of transistor Q1 is coupled to the emitters of a first transistor pair Q3 and Q4, and the collector of transistor Q2 is coupled to the emitters of a second transistor pair Q5 and Q6.
A balanced LO signal (LO+ and LOxe2x88x92) is also input to the circuit. LO+ is applied to the base of transistors Q3 and Q6, and LOxe2x88x92 is applied to the base of transistors Q4 and Q5. Consequently, the LO+ and LOxe2x88x92 signals switch their respective transistors on and off. The collectors of transistors Q3 and Q5 are coupled to the IF+ output and the collectors of transistors Q4 and Q6 are coupled to the IFxe2x88x92 output.
It is well known that the Gilbert cell of FIG. 1 provides an output (IF) that has components at frequencies equal to both the sum of and the difference between the input signal frequencies (LO and RF). Consequently, the signal processed by the receiver can be significantly lower in frequency than the received signal.
The traditional Gilbert cell, as shown in FIG. 1, for example, has a disadvantage in that has relatively high noise degradation. It is not unusual for the single-sideband noise figure of such a Gilbert cell to be over 9.0 dB.
The Gilbert cell shown in FIG. 1 also has another disadvantage in that it requires an input balun 10. On-chip input baluns are difficult to implement because they have high insertion losses and thus increase noise. On the other hand, off chip input baluns increase the cost and complexity of the mixer.
Consequently, there is a need for mixer having a single ended RF input, which would eliminate the need for an input balun. In addition, there is a need to reduce the noise of the mixer. Single ended RF input mixers used to date, however, while eliminating the need for an input balun and high-noise differential input transistors, typically have high local oscillator feedthrough and high second order nonlinearity. Consequently, there is a need for a single ended, low noise mixer having low local oscillator feedthrough and low second order nonlinearity.
A low noise double-balanced mixer circuit is provided that, in its downconversion embodiment, receives a single-ended RF signal and differentially mixes it with a local oscillating signal.
The RF signal is input to the base of a common-emitter transistor. The collector of the transistor is coupled to the primary winding of a transformer. The primary winding is RF coupled to ground via a capacitor. The transformer has two secondary windings that convert the single-ended RF signal from the primary winding into a balanced signal.
A first differential pair of transistors has their emitters coupled together. The emitters are coupled to a secondary winding of the transformer to receive one phase of the balanced RF signal (e.g., non inverted phase). A second differential pair of transistors also has their emitters coupled together. The emitters are coupled to the other secondary winding of the transformer to receive the other phase of the balanced RF signal (e.g., inverted phase).
The base of the first transistor of the first pair is coupled to the base of the first transistor of the second pair. These bases, in turn, are coupled to a first phase of a balanced local oscillating signal. The base of the second transistor of the first pair is coupled to the base of the second transistor of the second pair. These bases, in turn, are coupled to the second, inverted phase of the balanced local oscillating signal.
The collector of the first transistor of the first pair is coupled to the collector of the second transistor of the second pair. These collectors provide a first intermediate frequency output. The collector of the second transistor of the first pair is coupled to the collector of the first transistor of the second pair. These collectors provide a second intermediate frequency output.
Preferably, the primary winding of the transformer is DC connected to the secondary windings of the transformer. This provides a bias for the two differential pairs of transistors from the RF input transistor.
Consequently, the mixer of the present invention receives a single-ended RF input signal that is amplified via a low noise common-emitter transistor and converted to a balanced signal via a transformer. It is then differentially mixed with a balanced, local oscillating signal. The mixer outputs an intermediate frequency signal having frequency components consisting of the sum and differences of the LO and RF frequencies.
The mixer of the present invention is advantageously easy to implement as an integrated circuit. Moreover, since the mixer of the present invention has a single-ended RF input, there is no need for an internal or external input balun, thereby reducing the complexity of the chip and reducing noise. Moreover, since there is no need for differential input transistors, the noise of the mixer of the present invention is significantly reduced. It is anticipated that single-sideband noise figures under 5.0 dB can be achieved with the present invention.